This invention relates to buckles for safety belts used for releasably fixing the human body to the seat of vehicles such as automobiles. More particularly, this invention relates to a buckle of the type which is fixed to one end of one of a pair of safety belt straps in an automobile so that starting of the engine of the automobile by the ignition key cannot take place until a tang plate secured to the free end of the other safety belt strap is inserted completely into the buckle to actuate a switch in the buckle by driving a slider mounted slidably within the buckle, thereby an electrical signal for displaying complete wearing of the safety belt on the human body is generated and supplied to an alarm means incorporated in the dashboard or thereby an associated ignition switch circuit is closed.
Prior art buckles of this type for use with the safety belts have been defective in that the switch structure is quite complex and the large overall size results in a high manufacturing cost.